Conventionally, epoxy resins, polyester resins, polyacrylic resins, polyvinyl chloride resins, polyethylene resins and the like have been used as a resin for powder coating. In recent years, thermosetting resins have been mainly used from the points of physical properties and smoothness of a coating film. However, the thermosetting resins has the defect that where condensation resins are used or blocked isocyanate is used as a curing agent, condensation elimination components, blocking agent, and the like generate as a gas in curing, and bubbles tend to generate in a coating film.
Polyglycidyl compounds having no elimination component in the curing reaction are raised as a preferable curing agent used in a powder coating. However, where general glycidyl compounds, for example, bisphenol-type diglycidyl ether, is used as a curing agent, phenyl-glycidyl ether bond tends to decompose by light or heat. Further, where glycidyl ester, for example, diglycidyl terephthalate, is used as a curing agent, there is a problem of stability to water. Also, where glycidyl amide, for example, triglycidyl isocyanurate, is used as a curing agent, there is still insufficient point because requirement to weathering resistance becomes more severe in recent years.
Further, since the triglycidyl isocyanurate has high crystallinity, its melting point is as high as the range of 100 to 140.degree. C. For this reason, in order to uniformly kneading with a resin containing a carboxyl group, it is necessary to mix at high temperature to a certain extent (temperature approaching to the melting point). However, this triglycidyl isocyanurate has high reactivity, curing reaction of carboxyl group with glycidyl group somewhat proceeds even at a temperature in kneading, flowability (fluidity) slightly decreases when forming a powder coating and baking (when thermosetting), and there is the case that rough surface called orange peel occurs on a coating film surface.
On the other hand, tris(.beta.-methylglycidyl)isocyanurate has a melting point of 70 to 100.degree. C., and there may not occur the foregoing problem. However, as described in Japanese Patent Application Laid-open No. Hei 8-325481, tris(.beta.-methylglycidyl)isocyanurate has extremely high self-polymerization due to ring-opening polymerization, and some self-polymerization occurs even with use of the ring-opening polymerization inhibitor, so that more than stoichiometric tris(.beta.-methylglycidyl)isocyanurate may be required. In this case, hydrophilic characteristic derived from the isocyanurate ring causes water resistance to deteriorate.
On the other hand, as the technique using .beta.-methylglycidyl ester compound, a thermosetting resin composition comprising .beta.-methylglycidyl ester compound and an amine or a polycarboxylic acid are disclosed in British Patent No. 1,266,066.
Japanese Patent Application Laid-open No. Sho 49-24248 discloses the technique of utilizing polyglycidyl ester and/or poly .beta.-methylglycidyl ester of an aromatic polyvalent carboxylic acid having a melting point of 30 to 250.degree. C. including two or more glycidyl ester groups and/or .beta.-methylglycidyl ester groups as a curing agent in a molecule, particularly aiming at a storage of a powder.
International Publication No. WO93/04122 discloses a composition comprising a carboxyl group-containing polyester resin, an epoxy curing agent such as triglycidyl isocyanurate, methyl substituted triglycidyl isocyanurate or diglycidyl terephthalate, and a phosphonium salt catalyst.